"Our goal is to purchase sustainable beef by 2016," wrote Becca Hary, a director of global media relations at McDonald's, in an email. "Some areas of the world are more advanced when it comes to traceability capabilities, and it is likely that our first purchases will come from one of those areas. We will focus on increasing the annual amount each year, but it is too early to predict the quantity that will be purchased in 2016.

Hary added that it's too soon to predict if or how the move will impact beef prices in the future.

Eventually, McDonald's says, it wants to source all of its beef from sustainable sources. But even starting that process is difficult.

"This sounds simple, but it's actually a big challenge because there hasn't been a universal definition of sustainable beef," according to a McDonald's post on the topic.

After collaborating with a variety of food-industry stakeholders, including Cargill and the World Wildlife Fund, since 2011, the group has drafted a list of principles for sustainable beef. Still, no definition of the term exists, yet.

"We want to do our part to improve environmental practices in the way beef is produced, support positive workplaces in the beef industry,and drive continuous improvement in animal health and welfare. Beef represents our largest raw material purchase annually, and beef is our top priority," Hary wrote.

After McDonald's calculated its carbon footprint last year, it found that about 28 percent of its greenhouse gas emission impact is related to the beef in its supply chain, it said.

"Even though our purchases represent less than 2 percent of the total beef and dairy industry, we are working with other end users and the broader beef industry to address this important topic," the site noted.

McDonald's announcement comes as environmental sustainability remains a top trend in the culinary landscape. According to a recent survey of nearly 1,300 chefs by National Restaurant Association, "environmental sustainability" is forecast to be the third hottest trend in 2014.